1. Field of the Invention
The present invention relates to an absolute position detection method for a stroke sensing cylinder, and more particularly, to an improved absolute position detection method for a stroke sensing cylinder which is capable of detecting a direction and absolute position by sensing a stroke of a hydraulic and/or pneumatic cylinder (hereinafter called "cylinder"), which is used for automation of construction equipment, such as an excavator and wheel loader.
2. Description of the Conventional Art
Generally, in construction equipment (apparatus), such as an excavator, a cylinder is installed for driving a working apparatus, such as a boom, arm, bucket, etc. As operational oil is supplied to the cylinder and is discharged to a storing tank, the cylinder performs a stroke operation, thereby operating the working apparatus.
In the above-described construction equipment, as electric-hydraulic servo techniques have advanced, there is an increasing need for the apparatus to perform various operations independent of the skill of the operator.
Therefore, in order to automate the construction equipment, it is necessary to detect information concerning the position of the working apparatus. This information is needed for a control system, namely, for determining the relative position of the boom, arm and bucket as well as the absolute position thereof.
In order to detect information related to the position of the working apparatus, a position detection circuit is used for detecting the stroke of the cylinder, as shown in FIGS. 1 and 2.
FIG. 1 illustrates a schematic view of a conventional hydraulic and pneumatic cylinder which includes a cylinder rod having a magnetic scale.
As shown therein, the conventional hydraulic and pneumatic cylinder includes a piston 120 installed within a cylinder 110. The piston 120 may reciprocate in the up and down direction and the leftward and rightward direction. A rod 130, which supports the movement of the piston 120, is integrally formed with the piston 120 at one side thereof.
In addition, a magnetic scale 140 is formed at one side of the rod 130. The magnetic scale 140 has a protruded and recessed shape formed between the top dead point and the bottom dead point of the piston 120. A magnetic sensor 150, which is spaced-apart from the portion where the magnetic scale 140 is formed, is provided in the rod 130 for detecting a magnetic flux, wherein the magnetic flux varies based on the protruded and recessed portions. The magnetic sensor 150 produces a signal indicative of the magnetic flux. The magnetic sensor unit 150 is composed of a pair of sensors (sensor A and sensor B), such as a hall effect sensor, etc.
The cylinder rod 130 is made of steel (Fe), and the portion where the magnetic scale 140 is formed is coated with Cr. When the magnetic flux generated by the magnetic sensor 151 passes through the protruded and recessed portions, the output signal from the magnetic sensor unit 150 varies. Actually, the output signals have a sine wave form.
Furthermore, the distance between a pair of magnetic sensors is longer than one period of the magnetic scale 140.
In the thusly constituted conventional cylinder 110, as the piston 120 reciprocates between the top dead point and the bottom dead point of the cylinder 110, the rod 130, which is integrally engaged with the piston 120, is moved, and the moving state of the magnetic scale 140 formed on the rod 130 is detected by the magnetic sensor unit 150, thus recognizing the stroke position of the cylinder 110.
When the piston 120 is stopped, the magnetic scale 140 is detected by the magnetic sensor 151. The stroke variation of the cylinder 110 is then measured by computing the moving distance of the piston 120 based on the scale of the magnetic scale 140.
FIG. 2 is a block diagram illustrating a position detection circuit for a hydraulic and/or pneumatic cylinder using a 1/N dividing counter and a microprocessor.
As shown therein, the cylinder driving unit 210 is driven in accordance with a cylinder driving signal. The cylinder driving unit 210 in turn drives the cylinder 220 in which a magnetic scale is formed. The magnetic sensor unit 230 engaged with the cylinder 220 detects a magnetic variation of the magnetic scale and applies an output signal to a first microprocessor 240.
The microprocessor 240 communicates with a memory apparatus 150 and a 1/N divider counter 260, so that it controls the entire operations of the position detection switch with respect to the stroke of the cylinder, including a signal processing.
The operation of the position detection circuit for detection of the stroke of the cylinder will be explained with reference to the accompanying drawings.
The cylinder driving unit 210 drives the cylinder 220 in accordance with a cylinder driving signal inputted by an operator. A pair of magnetic sensors 231, such as a hall effect sensor, is installed in the magnetic sensor unit 230. The pair of magnetic sensors 231 detect the magnetic flux variation with respect to the magnetic scale (refer to reference numeral 140 of FIG. 1) formed in the rod of the cylinder 220, and the thusly detected signals are applied to the signal processing unit 232.
The signal processing unit 232 amplifies and filters the detection signals (sine wave forms) from the magnetic sensors 231. In other words, the signal processing unit 232 converts the detection signals into signals which are capable of being recognized by the microprocessor 240 and then the thusly converted signals are outputted to the microprocessor 240.
The microprocessor 240 converts the analog signals from the signal processing unit 232 into digital signals using the analog/digital converter. The microprocessor 240 further converts the sine wave form signals into square wave form signals using a predetermined algorithm.
The 1/N divider counter 260 receives the square wave form signals from the microprocessor 240 and divides it by 1/N and outputs the thusly divided signals to the microprocessor 240. The microprocessor 240 computes the stroke of the cylinder using the divided signals.
The detected pulse signal is divided to increase the accuracy of the detection by N-times. The value N is determined depending on the desired accuracy. The first microprocessor 240 counts the number of pulses, which is N-times the number of magnetic scales formed on the cylinder rod 130. The first microprocessor then computes the variation of the cylinder, and stores the thusly computed variation into the memory apparatus 250.
The values stored in the memory apparatus are displayed on a predetermined display unit.
The moving direction of the cylinder is determined by comparing a pair of square wave form phases. If the phase of the magnetic sensor B of the magnetic sensor unit 230 is ahead of the other phases, the cylinder rod shown in FIG. 1 moved in the direction of decompression (hereinafter called "normal direction"). If the phase of the magnetic sensor A is ahead of the other phases, the cylinder rod is moved in the direction of compression (hereinafter called "reverse direction").
FIG. 3 illustrates wave form diagrams of a signal processing 1/N counter when detecting a magnetic variation, based on the circuit of FIG. 2. When the cylinder is moved in the reverse direction, four pulses are generated depending on the following equation 1 (which will be described later), When the cylinder is moved in the normal direction, four pulses are generated depending on the equation 2 (which will be described later).
In Equations 1 and 2, A and B denote square waves converted from the sine waves detected by the magnetic sensors A and B of the magnetic sensor unit 230. /A and /B denote the inverted signals of A and B, and )A and )B denote wave forms generated by one shot circuit having the 1/4-dividing counter. Lastly, )/A and )/B denote the inverted signals of )A and )B. EQU (Ax)B)+(/Bx)A)+(Bx) /A)+(/Ax) /B) Equation 1 EQU (Ax) /B)+(/Bx) /A)+(Bx)A)+(/Ax)B) Equation 2
In the above-described conventional position detection apparatus for a cylinder stroke, since the pulses are detected by the magnetic scales formed in the protruded and recessed shape, only the relative position is detected. In addition, it is impossible to detect the absolute position from the magnetic variation of the initial position of the cylinder rod during the operation.
Additionally, the conventional position detection apparatus requires two magnet sensors to be used, wherein the sensors have a phase difference of 90.degree.. In this case, it is difficult to accurately position the sensors to have a phase difference of 90.degree. due to the assembling error.
Furthermore, the output sine wave form of the sensor may not maintain a consistent wave form due to external variables, such as vibrations, or impacts. Therefore, it is difficult to have a phase difference of 90.degree. with respect to the wave forms.
Therefore, there may occur a variation error. When such variation errors are accumulated, the accuracy of the detection of the relative position is decreased, and more seriously the direction of the stroke of the cylinder may be changed.